Conventionally, ship navigation assisting devices are known, which track ships based on information from a radar, obtain and display vectors of movements of the ships, and obtain a collision risk between one of the ships and a target ship (second ship) and a collision risk range. JP2012-021947A, JP1976-032475A and JP2786809B disclose such type of devices.
The navigation assisting device of JP2012-021947A causes an operation unit to calculate Closest Point of Approach (CPA) and Time to CPA (TCPA) at each given location on an estimated course, by using a detected target object image obtained by a radar antenna part, first-ship information, second-ship information, and estimated course information. The navigation assisting device further determines based on a result of the calculation (based on an estimated collision position) whether an alarm is required, and causes a display unit to display a risk symbol according to a result of the determination.
The operation evaluating device of a movable body (particularly a ship) in JP1976-032475A displays relative positions of an estimated path of the ship (first ship) to a collidable point with a target ship (second ship) and a collidable region centering on the collidable point, the collidable point being calculated based on coordinates and velocity component data of the target ship with respect to the first ship.
The ship navigation assisting device of JP2786809B obtains vertexes of a polygonally-shaped safety navigation area set around a first ship and a collision point with a target ship based on a relative position and a velocity vector of the target ship and a speed of the first ship, and calculates, as first-ship mapping positions, positions of the first ship when the respective vertexes correspond to the collision point. The ship navigation assisting device connects each other the first-ship mapping position for each calculated collision point by a line segment, displays the range defined by the line segment as a collision risk range, and superimposes the first-ship position and one of a velocity vector and a target ship position thereon.
However, with the configuration of JP2012-021947A, regarding the estimated collision position, a risk of colliding with the approaching second ship is notified to a helmsman simply by a change in color of a symbol of the first ship on a display screen or an audio alarm. In other words, on the display screen of JP2012-021947A, only the information indicating the risk of collision with a currently-scheduled course of the first ship is displayed. As a result, when the helmsman performs a course change steering operation (to avoid the risk), it is difficult for him/her to determine whether to change a ship speed or a heading, and, in the case of changing the heading, which direction to change it to. Further, there is also a case where a plurality of second ships are in the risk of colliding with the first ship, and it may be difficult to grasp an order of risk occurrence with among the second ships in terms of time. In such a situation, the steering operation becomes more difficult.
In this regard, in JP1976-032475A and JP2786809B, the area where there is a risk of the first ship colliding with the second ship if the first ship enters (i.e., the collidable region in JP1976-032475A and the collision risk range in JP2786809B) are displayed. Therefore, the helmsman can easily steer the ship to avoid the risk by referring to the area.
However, both the collidable region in JP1976-032475A and the collision risk range in JP2786809B are obtained based on a course of the target ship and displayed on the course of the target ship. As a result, when a plurality of target ships exist, since each target ship has a different course from the other, the helmsman easily receives a disorganized impression from the plurality of displayed collidable regions (collision risk ranges), and it is not easy to grasp a priority order for the helmsman to focus on among the collidable regions (collision risk ranges). Therefore, it is difficult to say that the helmsman can suitably and accurately steer the ship to avoid the risk by using either of the configurations of JP1976-032475A and JP2786809B.